1. Field of the Invention
The present invention relates to a power supply apparatus whose input source is a commercial AC power supply, and an image forming apparatus including the power supply apparatus.
2. Description of the Related Art
There are widely used power supply apparatus, which are to be mounted in various electronic equipment, and receive power from a commercial AC power supply so as to output necessary DC voltages to be used in the electronic equipment. For example, electronic equipment such as a copying machine and a printer usually uses a power supply apparatus whose input voltage is an alternating-current (AC) 100 V system (for example, AC 100 V to 120 V) for Japan and North America, or an AC 200 V system (for example, AC 220 V to 240 V) for Europe. However, the AC 100 V system and the AC 200 V type can share a power supply plug of the same shape depending on a country or a region, and hence both of a 100-V socket and a 200-V socket may exist in a mixed manner. In such a case, a user may erroneously input an overvoltage; for example, the user may erroneously input an A voltage of 200 V to a power supply apparatus of an AC 100 V system.
As a solution to the problem, an overvoltage protection unit for a power supply apparatus that receives a commercial AC voltage and outputs a DC voltage has been proposed. For example, a power supply apparatus that receives an AC voltage from a commercial AC power supply, and supplies a voltage to at least one load in parallel is proposed in Japanese Patent Application Laid-Open No. 2000-134794. Japanese Patent Application Laid-Open No. 2000-134794 describes an overvoltage protection circuit in which a fuse of a rated current that can reliably melt with the current-carrying capacity of a varistor is used as an overcurrent shut-off unit, which is connected in series to the varistor serving as an overvoltage restricting unit. When a rectifying and smoothing circuit in the power supply apparatus needs a large-capacity smoothing capacitor, an electrolytic capacitor is generally used. In the following, an electrolytic capacitor located at a subsequent stage of a rectification circuit that rectifies an AC voltage input from the commercial AC power supply is particularly referred to as “primary electrolytic capacitor.” Japanese Patent Application Laid-Open No. H07-184373, for example, describes a technology of using a primary electrolytic capacitor in a power supply apparatus and addressing the service life of the primary electrolytic capacitor and management of the service life thereof. The power supply apparatus described in Japanese Patent Application Laid-Open No. H07-184373 is a switching power supply apparatus with a relatively large capacity, which is used in a system apparatus having both a control system and a drive system, such as a copying machine. A unit capable of prolonging the service life of the large primary electrolytic capacitor used in the rectifying and smoothing circuit and managing the service life of the primary electrolytic capacitor is proposed in Japanese Patent Application Laid-Open No. H07-184373. A power supply circuit that has an overvoltage protection circuit added to the related-art power supply circuit, and is configured to melt a fuse before the primary electrolytic capacitor opens when an overvoltage exceeding the input voltage range is applied, to thereby protect the power supply apparatus against the overvoltage is proposed in Japanese Patent Application Laid-Open No. 2006-288155.
The operation of opening an aluminum electrolytic capacitor which is used as the primary electrolytic capacitor is described. The aluminum electrolytic capacitor is configured to include an oxide coating film formed on the surface of a high-purity aluminum foil for an anode as a dielectric substance, an aluminum foil for a cathode, an electrolyte solution, and a separator (electrolytic sheet). When a voltage sufficiently exceeding a rated voltage is applied in the forward current direction of the aluminum electrolytic capacitor, a leak current increases rapidly, thereby generating heat. The heat generation decreases the withstanding voltage of the dielectric substance. When the dielectric substance dielectrically breaks down as a consequence, a large current flows drastically, and hence the internal pressure of the electrolytic capacitor rises in a short period of time. A pressure valve is provided to protect the electrolytic capacitor when the internal pressure of the electrolytic capacitor rises. When the pressure valve actuates (opens), the gasified electrolyte solution is discharged from the open pressure valve, and hence a further pressure is not applied to the electrolytic capacitor. The primary electrolytic capacitor used in the power supply circuit is often a component designed against an overvoltage. The component designed against an overvoltage is configured not to be short-circuited when a voltage equal to or higher than the withstanding voltage is applied and the pressure value actuates. That is, the pressure valve actuates safely. When the pressure valve opens, however, the discharged gasified electrolyte solution gives a distinctive smell. In this respect, an electrolytic capacitor with a high withstanding voltage or a component designed against an overvoltage is used, and an overvoltage protection circuit is further used to prevent the valve of the electrolytic capacitor from opening.
When a smaller and a lower-cost primary electrolytic capacitor is used in a power supply apparatus to cope with cost reduction and downsizing of the power supply apparatus, the withstanding margin of the primary electrolytic capacitor may become insufficient. In such a case, it is necessary to carry out more accurate detection of an overvoltage to prevent the valve of the primary electrolytic capacitor from opening. Further, to meet the specifications associated with the current power saving attempts, the circuit that detects an overvoltage needs to operate with low power consumption. When an overvoltage is detected accurately, however, the detection current generally needs to be large to a certain degree. This makes it difficult to detect an overvoltage with low power consumption.